1. Technical Field
This application is related to systems for modeling atmospheric turbulence, and more specifically, to systems for measuring and simulating atmospheric turbulence for testing adaptive optical systems.
2. Background Information
Light traveling from a star, or any point source, will propagate spherically outward. As shown in FIG. 1, after a long distance, the wavefront, or surface of equal phase, will be flat. The Earth's atmosphere is a large, non-linear, non-homogenous medium that is constantly changing in a random fashion that affects light as it propagates through it.
When the light begins to propagate through Earth's atmosphere, the varying indices of refraction will alter the optical path, as shown in FIG. 2. The Earth's atmosphere is a non-homogeneous continuous turbulent medium, and its properties vary based on factors including temperature, pressure, wind velocities, and humidity. The Earth's atmosphere also temporally and spatially changes in a random fashion. The Kolmogorov model for atmospheric turbulence is a description of the nature of the wavefront perturbations introduced by the atmosphere and it is one of the most accepted models. It is supported by a variety of experimental measurements and is quite widely used in simulations for exo-atmospheric seeing. The Kolmogorov theory of atmospheric turbulence is based on the assumption that the turbulence varies the index of refraction and thus affects the permittivity of the medium throughout it. This affects the light as it propagates through the atmosphere.
Adaptive Optics (AO) is a term used for a class of techniques dealing with the correction of wavefront distortions in an optical system. Some wavefront distortions may include those caused by the atmosphere. Exo-atmospheric applications of adaptive optics particularly include the correction of atmospheric turbulence for a telescope system. Adaptive optics techniques are used in free space laser communication systems, high energy laser systems, and phase correction for deployable space-based telescopes and imaging systems.
Prior to deployment, an adaptive optics system requires calibration and full characterization in the laboratory. Creating realistic atmospheric simulations has been notoriously difficult, computationally intensive, and expensive. Many techniques are currently being used with adaptive optics systems for simulating atmospheric turbulence. One static component technique uses glass phase screens with holograms etched into them. Another technique uses a rotating filter wheel with etched holographic phase screens to simulate temporal transitions. However, generating and manufacturing these holograms can be quite expensive. Other methods simulate atmospheric turbulence by using a static aberrator, such as a clear piece of plastic, and rotating it, or by simply using a hot-plate directly under the beam path to cause local turbulence.